Noise suppressing circuit



June 30, 1931. v. E. LEGG NOISE SUPPRESSING cmcum Filed May 14, 1929 SUBSCRIBE/Q15 5mr/0/v TELEPHONE L/NE wmvrox? VELsae BYj ATTORNEY Patented June 30, 1931 UNIT'E DlSTA-TES mm T VICTOR LEGG, F MAPLE- OOD, NEW JERSEY, assrGNo T0 BELL f'rnnnr ronn LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., a CORPORATION on NEW YORK NOISE SUPPRESSING CIRCUIT Application filed'May 14,

This invention relates to the problem of discriminating between currents of different magnitude, particularly in signaling systems such as telephone systems. The invent on has particular reference to the suppression of crosstalk currents or other noise currents which are of small magnitude relative to the signaling currents. o

Noiseor crosstalkismostly objectionable during pauses in conversation, the crosstalk currents being most evident during such pauses but beingless apparent when the relatively stronger speech currents are being transmittedand received. V

An object of the invention is to suppress the transmission of crosstalk currents or other small disturbing currents while" at the same time-freely permitting the'transmission of speech currents.

A feature of the invention is theuse of variable series impedances in av telephone line orsimilar system for the purpose of suppressing small noise currents such as crosstalk. These series impedances maybe any elements whose impedance is hlgh at the low applied crosstalk voltagesrandzlow at the relatively higher signal voltages. Specifically such elements 'may consist of static rectifier elements'such as crystals, copper oxide rectifiers etc;

"Rectifiers such-as crystals have been used in radio reception and for other purposes making use of the unsymmetrical transmita tingproperties which the crystal exhibits fordifferent polarities of current. They have also "beenvused in much the same-way as spark gaps and other types ofdischarge apparatus to protect circuitsfrom-excessive voltages by shunting-off the resultant large currents trom the circuit to be protected. "In this latter use of crystals it is notthe unsymmetrical transmitting characteristic 'fforvopposite polarities of current which is important but ratherthe non-linear voltageiinpedance characteristic which the crystal possessesfor applied voltages inthe direction corresponding to greatest current transmission. As is well known,for voltages in this direction, the impedance is higher at 1929 Serial ITO-362,903.

to be of the order of imilliamperes, acrosstalk suppressor to bemost eflicient at a level of 1000 crosstalk unitsmust discriminate between currents of the magnitudes of 4:.millii amperes and .OOimilliampe're. Applicant has found that :certain '-rectifiers particularlyofthe copper oxide-lead type, are ca,- pable of giving amarked discrimination "between currents of the small-ordersao-if magnitude mentioned-while at thesam'etimenot introducing too high-an impedance'into the series talking circuit.

In accordance with-the invention-one or more rectifiers are provided in seriesineach 5 side of the talking c1rcu1t for each direction of transmission thereover- The invention *will be more fully understood from the following specification in connection with the accompanying drawing of whichxthe single'figure shows insimple diagrammatic fO-rma subscribers telephone line equipped with crosstalk suppressors in accordance withthe invention. I

While the invention is applicableto different types ofsignaling systems and to telephoneflines of diiferent character, the drawing illustrates crosstalk suppressors in the subscribers loop. It will, of.course, -'be obvious to locate the suppressors elsewhere in the system, for example, at a .central oflice station or in atoll line. v Referring to the drawing,the subscribers line 10 is shownleading-to subscribersstation -l1 WhlCh is not illustrated 'in 'detail since it may be of any; desired type, dependent. invno way uponthe prese'nt invention. Crosstalk-suppressor" circuits 12 are shown included in series in each side otthe tele phone line -10, These suppressors are disclosed as comprising a number'of rectifiers 13 connected in series for each direction of transmission over the conductor in which they are included. It is within the invention to insert the suppressors directly in the series talking circuit but transformers such as 14 and 15 may be used where the existing terminal impedances are not of the proper magniude, the purpose of these transformers being to transform the impedances between which the suppressor circuits operate to the appropriate values for most eliective operation. Instead of changing the effective terminating impedances to suit the rectifier characteristics, rectifiers may, of course, be chosen of the proper characteristics to suit the existing terminating impedances where possible.

Three rectifier elements have been shown connected in series in each transmission path throughthe suppressor circuits but the number of such elements may vary to suit the different line and transmission conditions and depending also on the size of the individual rectifiers.

Applicant has employed a type of copper oxide rectifier known to the trade as Kuprox rectifier, type A-10, with a rated current capacity of .8 to 1.0 ampere. \Vith three of these connected in series in each current transmitting path as shown in the drawing, applicant has obtained a discrimination in transmitted energy of the order of 3 db. 'to 8 db. between currents of the order of magnitude of 4 milliamperes and a few thousandths of a milliampere respectively. The number of db.s, or decibels, is a logarithmic measure of the ratio of the powers represented by the respective currents and is determinedquantitively by the expression:

N log gl- The order of 3 db. discrimination was obtained when the suppressor circuits operated between terminating impedances of about 600 ohms whereas the order of 8 db. discrimination was obtained where the terminating impedances were 100 ohms. Intermediate amounts of discrimination were obtained by terminating impedances intermediate those given. The loss of currents of the 4 milliampere level was computed as 1.8 db. for the 600 ohm termination and .6 db. for the 100 ohm termination.

It will be understoodthat the invention is not to be construed as limited to the particular type of rectifier mentioned above nor to the particular number disclosed nor their magnitudes but it is within the scope and spirit of the invention to use equivalent transmission elements whose impedance changes in inverse relation tothe applied voltage at the necessary levels and. by the necessary extent to suppress crosstalk or similar currents while transmitting the speech currents.

What is claimed is:

1. The combination with a telephone line of means for suppressing crosstalk currents while transmitting speech currents, said means comprising resistive material in series in the line, the impedance of which decreases with increasing applied voltages.

2. The combination with a telephone line of a resistance element in series in said line of high impedance to currents of crosstalk levels, the impedance of said element decreasing appreciably with increasing applied voltages immediately above the voltage range of crosstalk currents.

3. The combination in a telephone circuit of a series element whose resistance varies in inverse manner to the applied voltages,

said element having a low impedance fornormal speech currents traversing the line' and relatively high impedance to currents of crosstalk magnitude.

4. The combination defined in claim 3 in which a separate element is provided in each b side of the telephone circuit for each direction of transmission thereover.

5. The combination with a telephone circuit of unilaterally conducting material in each side of the circuit for each direction of current flow, and means for altering the efiective impedance presented by said material to the telephone circuit whereby said material efi'ectively discriminates against transmission of currents of crosstalk level ioo while transmitting normal speech currents.

6. A variable impedance for a telephone line comprising a copper oxide rectifier connected in series in each side of the line and dimensioned to transmit normal speech cur-- rents while suppressing currents of cross talk level.

7. A crosstalk suppressor circuit; for a telephone line comprising rectifying crystal elements connected to provide paths of free transmission for normal speech currents in both directions over the telephone line but only pathsof high impedance to currents of crosstalk level.

8. The combination in a telephone circuit 10. The combination with a circuit for the transmission of signal waves of a transmission element associated with said circuit comprising a crystal having the property of freely transmitting said signal waves practically undistorted while suppressing the transmission of disturbing currents having amplitudes small in comparison with said signal waves. I

11. In a current amplitude discriminating circuit for translating signals, discriminating means in said circuit comprising a crystal the impedance of which is nonlinear, said impedance being large to small currents but comparatively small to normal currents, and a work circuit utilizing currents transmitted through said discriminating means within the frequency range of the waves impressed on said discriminating means.

In witness whereof, I hereunto subscribe my name this 11th day of May 1929.

VICTOR E. LEGG. 

